Nowadays, some image capturing apparatuses having a solid-state image sensor such as a CCD or CMOS sensor have a so-called live view function that allows the user to confirm an object image by sequentially outputting image signals continuously read out from the image sensor to a display device arranged on, for example, a back surface of a camera.
As general methods using a light beam that passes through an imaging lens of automatic focus detection/adjustment methods of image capturing apparatuses, a contrast detection method (called a defocus detection method) and phase difference detection method (called a shift detection method) are available. The contrast detection method is popularly used in video movie apparatuses (camcorders) and digital still cameras, and uses an image sensor as a focus detection sensor. This method focuses attention on an output signal from an image sensor, especially, information of high frequency components (contrast information), and decides a position of an imaging lens where an evaluation value of that information is maximized as an in-focus position. However, this method is not suited to a high-speed focus adjustment operation since it is also called a hill-climbing method, that is, since it calculates an evaluation value while moving an imaging lens by a small amount and is required to move the lens until a maximum evaluation value is consequently detected.
On the other hand, the phase difference detection method is popularly used in single-lens reflex cameras using silver halide films, and is a technique which most contributes to the practical use of AF (Auto Focus) single-lens reflex cameras. In the phase difference detection method, a light beam that passes through an exit pupil of an imaging lens is split into two beams, which are respectively received by a pair of focus detection sensors. Then, by detecting a shift amount between signals output according to the light-receiving amounts, that is, a relative positional shift amount in the splitting direction of the light beam, a shift amount of a focus direction of the imaging lens is directly calculated. Therefore, once the focus detection sensors perform accumulation operations, a focus shift amount and direction can be obtained, thus allowing a high-speed focus adjustment operation. However, in order to split a light beam that passes through the exit pupil of the imaging lens into two beams, and to obtain signals corresponding to the respective light beams, it is a general practice to arrange an optical path splitting means including a quick return mirror and half mirror in an imaging optical path, and to arrange a focus detection optical system and the focus detection sensors after the optical path splitting means. For this reason, the apparatus unwantedly becomes bulky and expensive. Also, in a live view mode, since the quick return mirror is retracted from the optical path, an AF operation is disabled, thus posing a problem.
In order to solve the above problem, a technique which gives a phase difference detection function to an image sensor to obviate the need for dedicated AF sensors, and to implement a high-speed phase difference detection, AF operation has been proposed. For example, in Japanese Patent Laid-Open No. 2000-156823, a pupil splitting function is given by decentering sensitive areas of light-receiving portions from optical axes of on-chip microlenses in some light-receiving elements (pixels) of an image sensor. Then, these pixels are used as focus detecting pixels, and are arranged at predetermined intervals in an image forming pixel group, thus attaining focus detection based on the phase difference detection method. Also, since positions where the focus detecting pixels are arranged correspond to deficient portions of image forming pixels, image information is constructed by interpolation from pieces of surrounding image forming pixel information.
In Japanese Patent Laid-Open No. 2000-292686, a pupil splitting function is given by splitting light-receiving portions of some pixels of an image sensor. Then, these pixels are used as focus detecting pixels, and are arranged at predetermined intervals in an image forming pixel group, thus attaining focus detection based on the phase difference detection method. With this technique as well, since positions where the focus detecting pixels are arranged correspond to deficient portions of image forming pixels, image information is constructed by interpolation from pieces of surrounding image forming pixel information.
However, in Japanese Patent Laid-Open Nos. 2000-156823 and 2000-292686 above, since pieces of image information for the positions where the focus detecting pixels are arranged as the deficient portions of image forming pixels, they are constructed by interpolation from surrounding image forming pixels, correct interpolation often fails depending on objects. For this reason, when the number of focus detecting pixels is sufficiently smaller than the number of normal image forming pixels, image quality deteriorates slightly. However, with increasing ratio of focus detecting pixels, the image quality deteriorates more seriously.
As is known, in order to attain a target frame rate in a live view mode, since pixel signals have to be read out from the image sensor at high speed, they are read out at high speed while thinning out some pixels in the image sensor. In this case, when pixels are arranged to include focus detecting pixels in readout pixel signals so as to allow an AF operation even in the live view mode, the ratio of focus detecting pixels to image forming pixels increases compared to a case in which all pixels are read out, thus influencing the image quality more seriously.
In general, a CMOS solid-state image sensor is manufactured via a plurality of mask processes. Since the manufacture is done while aligning positions between respective mask processes, positional deviations have occurred between members manufactured in early processes and those manufactured in latter processes. Since photo-electric conversion units of the solid-state image sensor are manufactured in the early processes and microlenses are formed in the last process, positional deviations often have normally occurred between the photo-electric conversion units and microlenses. For this reason, vignetting occurs depending on the positions of focus detecting pixels, thus disturbing accurate focus detection.